The present invention relates to a conductive paste used for improving fine pattern printing properties during the printing process of ceramic thick film printed circuit board.
In recent years, electronic components have been required to reduce in size and weight to accommodate to mobile phones and compact office automation devices. In the field of ceramic thick film printed circuit board, lamination methods including the green sheet laminating method have been utilized to meet the needs of high-density printed circuit boards such as micro chip module (MCM) and chip size package (CSP).
In the chip component field as well, highly multi-layered electronic components such as chip capacitors and chip inductors have been used widely in order to satisfy the need to reduce the size of chips. In the manufacturing process of chip resistors and other screen printing processes, introduction of high definition screen masks and pastes with superior printing properties have promoted fine pattern printing.
Chip components manufactured by using the intaglio transfer printing technology capable of fine printing have been commercialized. Against this background, the number of printing processes has increased for the manufacturing of multiple laminated components used for highly-density printed circuit boards. Furthermore, the intaglio transfer printing method is costly since it requires expensive materials such as intaglio.
Conventional thick film conductive pastes are manufactured by mixing powders of conductive materials (precious metal materials) and inorganic binders (glass frit and additives) with an organic vehicle made by dissolving ethyl cellulose or the like resins in an organic solvent such as xcex1-terpineol , and by kneading the mixture to form a paste. When the powder and the organic vehicle are mixed, viscous lumps of 30-50 xcexcm "PHgr" are generated due to a lack of dispersion.
In conventional screen printing methods, the width of wiring has been typically between 250-300 xcexcm. For a screen mask, a stainless steel screen of 200-325 mesh has been used. The screen mask of 200-325 mesh has openings of 50-80 xcexcm, thus the lumps caused by the lack of dispersion mentioned above have not caused any problems in terms of passing through the mesh.
In finer screen printing of recent years, a stainless steel mesh of 400-600 mesh has been required to form a wiring of 100 xcexcm in width, reducing the size of the openings of the mesh to 27-40 xcexcm "PHgr". Due to this, passing of the lumps caused by the insufficient dispersion has been, in some cases, hampered, allowing the lumps to remain on the stainless steel mesh without being printed, thereby making the printed pattern defective.
With reference to FIGS. 3A and 3B, the aforementioned phenomenon is described. FIGS. 3A and 3B are schematic views in which patterns are printed with conductive paste using screen masks. As FIG. 3A shows, when paste disperses well, a cross section of the conductive paste printed on a substrate 21 by screen printing, is like a printed layer 24. In other words, when the paste disperses well, hardly any paste remains on a screen mesh 22 and a side wall of a masking emulsion 23.
As FIG. 3B shows however, when lumps are generated in the paste due to insufficient dispersion, lumps 25 of the conductive paste block the screen mesh 22 during screening printing, causing a defective recess in the printed layer 24. When a defective recess 26 grows larger, it triggers breaking of conductive film.
As thus far described , in the case of the conventional pastes such as Agxe2x80x94Pd and Agxe2x80x94Pt pastes containing fine particles of Pd or Pt of smaller than 0.5 xcexcm "PHgr", fine particles do not disperse sufficiently, causing lumps of 30-50 xcexcm in size, which are more viscous than the paste, to be generated. Even when additives such as inorganic binders including bismuth oxide , copper oxide and zinc oxide for making the printed conductive film more tightly fixed to the substrate are sieved by a mesh, particles with larger diameters can not be removed. Thus those particles block the screen mask during screen printing. When a screen mask of 400-600 mesh is used during screen printing to form a wiring of not more than 100 xcexcm in width, the lumps in the paste, caused by insufficient dispersion or inorganic binders containing particles of larger diameter are trapped in the openings of the mesh of the screen mask, causing a defect in the printed layer.
As an alternative paste, metallic resinates, thin-film forming pastes containing organic acid salts of precious metals, have been conventionally used. These resinates are produced by reacting , for example, rosin (resin of pine trees) with metallic chlorides. These pastes do not contain solid powder and, thus do not block the mesh during printing. However, the film formed with those pastes is only about 0.1 xcexcm in thickness and, thus can not be used for common thick film printed circuit board.
The present invention aims at providing a conductive paste for fine pattern printing used in the printing process of ceramic thick film printed circuit boards. Conductive particles used in the conductive paste of the present invention are approximately spherical. The conductive paste further includes organic acid salts of precious metal instead of fine particles of Pd or Pt which do not disperse sufficiently when used in a conductive paste, in order to improve the passing of the paste through high-mesh screen masks.
According to the present invention, precious metal organic acid salts and precious metal powder are both used as precious metal components of a conductive paste. The base metal organic acid salts are also used as base metal components in the conductive paste in order to exclude particles with larger diameters from the conductive paste. The conductive paste of the present invention allows the lumps in the paste caused by the insufficient dispersion to be not more than 10 xcexcm "PHgr", thereby remarkably reducing the blocking occurring during screen printing of wiring pattern of 100 xcexcm or less.